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Document the Shares option in Memory panel

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Emmanuel Kasper 2016-06-09 13:26:42 +02:00 committed by Dietmar Maurer
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@ -252,6 +252,18 @@ done via a special `balloon` kernel driver running inside the guest, which will
grab or release memory pages from the host.
footnote:[A good explanation of the inner workings of the balloon driver can be found here https://rwmj.wordpress.com/2010/07/17/virtio-balloon/]
When multiple VMs use the autoallocate facility, it is possible to set a
*Shares* coefficient which indicates the relative amount of the free host memory
that each VM shoud take. Suppose for instance you have four VMs, three of them
running a HTTP server and the last one is a database server. To cache more
database blocks in the database server RAM, you would like to prioritize the
database VM when spare RAM is available. For this you assign a Shares property
of 3000 to the database VM, leaving the other VMs to the Shares default setting
of 1000. The host server has 32GB of RAM, and is curring using 16GB, leaving 32
* 80/100 - 16 = 9GB RAM to be allocated to the VMs. The database VM will get 9 *
3000 / (3000 + 1000 + 1000 + 1000) = 4.5 GB extra RAM and each HTTP server will
get 1/5 GB.
All Linux distributions released after 2010 have the balloon kernel driver
included. For Windows OSes, the balloon driver needs to be added manually and can
incur a slowdown of the guest, so we don't recommend using it on critical